The Effect of the XM45 Gas Mask and Hood on Directional Hearing

Abstract

We measured listeners' directional sensitivity for narrow bands of noise and for speech recognition while they were wearing the XM45 gas mask and hood and compared it to normal free-field listening. Bekesy tracking was used to measure absolute hearing thresholds for 1/3 octave bands of noise centered at 0.125, 0.25, 0.5, 1, 2, 3, 4, 6, and 8 kHz. and masked intelligibility thresholds for speech were measured using the Modified Rhyme Test. Both the noise and speech signals were presented from loudspeakers located at azimuths of 0°, 45°, 90°, 135°, and 180°. The mask and hood significantly elevated noise-band thresholds for those bands centered at 1.0 kHz and above. Sensitivity to masked speech was increased by over 12 dB by lateral loudspeaker placement, and wearing a gas mask and hood reduced this gain by 80 percent. These results document a loss of auditory spatial awareness that has not been apparent from previous studies of speech intelligibility while wearing chemical protective equipment.

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